Is this just because it's a hatch vs a coupe style hatch like the RSX? I can't imagine it's because the Civic has more "accessories" or a stiffer chassis than the RSX. What's the deal?

Civic Si = 26/31mpg
RSX = 27/33 mpg
RSX-S = 24/31 mpg

Civic Si = 2782 lbs
RSX = 2694 lbs
RSX-S = 2767 lbs